20 research outputs found
Timescales of carbon turnover in soils with mixed crystalline mineralogies
Organic matter–mineral associations stabilize much of the carbon
(C) stored globally in soils. Metastable short-range-order (SRO) minerals
such as allophane and ferrihydrite provide one mechanism for long-term
stabilization of organic matter in young soil. However, in soils with few SRO
minerals and a predominance of crystalline aluminosilicate or Fe (and
Al) oxyhydroxide, C turnover should
be governed by chemisorption with those minerals. Here, we correlate mineral
composition from soils containing small amounts of SRO minerals with mean
turnover time (TT) of C estimated from radiocarbon (<sup>14</sup>C) in bulk soil,
free light fraction and mineral-associated organic matter. We varied the
mineral amount and composition by sampling ancient soils formed on different
lithologies in arid to subhumid climates in Kruger National Park (KNP), South
Africa. Mineral contents in bulk soils were assessed using chemical
extractions to quantify Fe oxyhydroxides and SRO minerals. Because of our
interest in the role of silicate clay mineralogy, particularly smectite
(2 : 1) and kaolinite (1 : 1), we separately quantified the mineralogy of
the clay-sized fraction using X-ray diffraction (XRD) and measured <sup>14</sup>C
on the same fraction.
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Density separation demonstrated that mineral associated C accounted for
40–70 % of bulk soil organic C in A and B1 horizons for granite,
nephelinite and arid-zone gabbro soils, and > 80 % in other
soils. Organic matter strongly associated with the isolated clay-sized
fraction represented only 9–47 % of the bulk soil C. The mean TT of C
strongly associated with the clay-sized fraction increased with the amount of
smectite (2 : 1 clays); in samples with > 40 % smectite it
averaged 1020 ± 460 years. The C not strongly associated with
clay-sized minerals, including a combination of low-density C, the C
associated with minerals of sizes between 2 µm and 2 cm (including
Fe oxyhydroxides as coatings), and C removed from clay-sized material by
2 % hydrogen peroxide had TTs averaging 190 ± 190 years in surface
horizons. Summed over the bulk soil profile, we found that smectite content
correlated with the mean TT of bulk soil C across varied lithologies. The SRO
mineral content in KNP soils was generally very low, except for the soils
developed on gabbros under more humid climate that also had very high Fe and
C contents with a surprisingly short, mean C TTs. In younger landscapes, SRO
minerals are metastable and sequester C for long timescales. We hypothesize
that in the KNP, SRO minerals represent a transient stage of mineral
evolution and therefore lock up C for a shorter time.
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Overall, we found crystalline Fe-oxyhydroxides (determined as the difference
between Fe in dithionate citrate and oxalate extractions) to be the strongest
predictor for soil C content, while the mean TT of soil C was best predicted
from the amount of smectite, which was also related to more easily measured
bulk properties such as cation exchange capacity or pH. Combined with
previous research on C turnover times in 2 : 1 vs. 1 : 1 clays, our
results hold promise for predicting C inventory and persistence based on
intrinsic timescales of specific carbon–mineral
interactions
Impact of rainfall and topography on the distribution of clays and major cations in granitic catenas of southern Africa
Soil catenas integrate and amplify gravity transfer and differentiation processes of eluviation and illuviation in soil profiles. We quantified differences in these redistribution processes along granitic catenas across an arid to sub-humid climate gradient in Kruger National Park, South Africa. We measured soil properties in nine catenas sampled from three areas receiving annual rainfall of 470 mm (arid zone), 550 mm (semi-arid zone) and 730 mm (sub-humid zone). As rainfall increased, kaolinite replaced smectite as the dominant clay mineral in all landscape positions across the catenas. Toeslopes showed the strongest evidence of this transition with an excess of smectite in the arid catenas but complete prevalence of kaolinite in toeslopes of sub-humid catenas. The concentration and distribution of clay along the catenas were dependent on landscape position as well - soil profiles at and near the crests were clay depleted (as low as 1%) while those at the toeslopes had much more clay (up to 60%). Clay redistribution along catenas was sensitive to climate with the least amount of redistribution occurring in the dry sites and the most occurring in the wet sites. As a consequence, the sub-humid catenas had clay accumulation only in a small part of the toeslopes while the bulk of their length was represented by highly leached soils. In contrast, arid zone catenas showed little clay redistribution and semi-arid sites displayed the greatest within-catena clay redistribution and preservation. Clay movement and storage conditioned other soil properties such as CEC, base cation distribution, base saturation and pH. (C) 2011 Elsevier B.V. All rights reserved
Chemical transfers along slowly eroding catenas developed on granitic cratons in southern Africa
catenas the redistribution of mass occurs predominantly as plasma, the dissolved and suspended constituents in soil water. We applied mass balance methods to track how redistribution via plasma contributed to physical and geochemical differentiation of nine slowly eroding (~5 mm ky−1) granitic catenas. The catenas were arrayed in a 3 × 3 climate by relief matrix and located in Kruger National Park, South Africa. Most of the catenas contained at least one illuviated soil profile that had undergone more volumetric expansion and less mass loss, and these soils were located in the lower halves of the slopes. By comparison, the majority of slope positions were eluviated. Soils from the wetter climates (550 and 730 mm precipitation yr−1) generally had undergone greater collapse and lost more mass, while soils in the drier climate (470 mm yr−1) had undergone expansion and lost less mass. Effects of differences in catena relief were less clear. Within each climate zone, soil horizon mass loss and strain were correlated, as were losses of most major elements, illustrating the predominant influence of primary mineral weathering. Nevertheless, mass loss and volumetric collapse did not become extreme because of the skeleton of resistant primary mineral grains inherited from the granite. Colloidal clay redistribution, as traced by the ratio of Ti to Zr in soil, suggested clay losses via suspension from catena eluvial zones. Thus illuviation of colloidal clays into downslope soils may be crucial to catena development by restricting subsurface flow there. Our analysis provides quantitative support for the conceptual understanding of catenas in cratonic landscapes and provides an endmember reference point in understanding the development of slowly eroding soil landscape
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Shaping post-orogenic landscapes by climate and chemical weathering
The spacing of ridges and valleys reflects the competition between diffusive transport on hill slopes and advective transport in channels, although the underlying lithologic, tectonic, and climatic controls have not been untangled. Here, we measure geochemical and geomorphic properties of catchments in Kruger National Park, where granitic lithology and erosion rates are invariant, enabling us to evaluate how varying rainfall (MAP=470, 550, and 730mm) impacts ridge-valley spacing. Catchment-average erosion rates, based on 10Be concentrations in river sands, are low (3 to 6 m My-1) and vary minimally across the three sites. Our lidar-based, slope-area trends reveal that hill slopes in the dry site are gentle (3%) and short (area of hill-valley transition is 75m2). The terrain has low relief and is highly dissected by small channels. With increasing rainfall, hill slopes lengthen (transition area increases from 220 to 700m2) and increase in gradient (6 to 8%), resulting in fewer, higher relief catchments. The chemical depletion fraction of ridge-crest regoliths increases with rainfall from 0.3 to 0.7, indicating progressive increase in chemical relative to physical erosion. In the dry site, catenas are relatively undifferentiated, but intermediate and wet site catenas have leached sandy crests and upper side slopes complemented by clay accumulation zones further down slope. The texture of this granitic landscape appears to be set by climate-driven feedbacks among chemical weathering, regolith fabric differentiation and hydrological routing that enhances the vigor of diffusive transport relative to advective transport
Inselbergs persist as islands of diversity in a heavily grazed rangeland mosaic at the nexus of three arid biomes
Inselbergs are regarded as ‘islands of diversity’ due to the high number of plant species present and level of endemism. They also act as natural sources of fodder for livestock and thus risk becoming homogenised in a heavily grazed rangeland. The aim of this study was to compare plant diversity on inselbergs and matrices inheavily grazed sections of three arid biomes of South Africa’s west. The inselbergs are scattered in a matrix of rangeland vegetation in the Desert, Nama-Karoo and Succulent Karoo biomes where pastoral herding is the main land use. Plant composition, cover and growth forms were quantified in plots and along transects in 21 inselberg and matrix sites. Inselbergs had more diverse plant and growth forms and greater cover than adjacent matrices. Defoliated plants on inselbergs are able to recover due to more mesic conditions and because inselbergs are largely grazed during drier periods, species are able to flower and set seed during winter. Inselbergs are also less accessible to livestock due to their unique geomorphology. Therefore, inselbergs are able to persist as islands of diversity and warrant inclusion into conservation and rangeland farming management plans since climate change will result in biome boundary shifts.Keywords: arid biomes, communal rangelands, inselberg
The composition of the soil seedbank and its role in ecosystem dynamics and rehabilitation potential in the arid Tankwa Karoo Region, South Africa
Very few soil seedbank studies have been conducted in South Africa, especially in arid rangelands. Insight into the soil seedbank could therefore improve assessment of rangeland dynamics and enhance rehabilitation efforts. This study aimed to characterise the soil seedbanks in various vegetation types of the Tankwa Karoo National Park, an arid environment in South Africa. At 43 sites soil was sampled twice at the end of spring and the end of autumn, and the composition and structure of aboveground vegetation were described at the peak growth period in spring. Seeds were isolated from the soil samples by means of flotation in a salt solution, seedbank species composition was obtained by seedling germination, and the vegetation was investigated using a line-point survey method. Overall seed density was 8 034 seeds m−2 of which 55% was viable, and the common life-forms were therophytes and chamaephytes. Species richness and diversity were lower in the seedbank compared with those of the vegetation, and the two vegetation spheres were 25% similar. While seedbank composition suggested poor veld condition, there was enough seed density and viability for future regeneration and rehabilitation initiatives. However, this may be impeded by the absence of many perennial species in the soil seedbank.Keywords: arid environment, interpatches, life-form spectra, patches, rangeland condition, species composition, Succulent Karo
Inselbergs persist as islands of diversity in a heavily grazed rangeland mosaic at the nexus of three arid biomes
Indicators of Economic Status and Social Capital in South African Townships
This article proposes a set of non-monetary indicators of material and social resources characterizing the childrearing environments of families in South African black urban townships. The selected indicators include adequacy of food, quality of housing, financial assets, consumer goods and social resources. The study reported in the article was carried out between 1990 and 1996. Through factor analyses two dimensions of social and economic status (SES) - consumption and social/financial capital - were identified. High, moderate and low SES groups were formed based on the families' combined score on the two factors. SES was significantly predictive of a family's ability to meet basic needs and in its experience of material hardship. The results of the study are reported.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66883/2/10.1177_0907568297004002005.pd